Aila,
I think you're assuming that the clitoris is more important among
promiscuous breeders?
Although humans certainly do qualify as promiscuous breeders, sexual
activity is still largely governed by hormonal activity. Females are more
sexually active (though not exclusively) near and at the time of
ovulation than at other times. As in other animals, peak activity is
still governed by a peak in reproductive "readiness".
I currently work with mares. Mares are seasonal breeders and will only
allow a stallion to mount them during estrus. They are by no means
promiscuous. On the other hand, they have a very large (in fact, huge),
well innervated clitoris and are capable, presumably, of "enjoying" sex
as humans can.
Certainly I would say that the clitoris "helps" as you suggest, but I
would suggest that the evolution of sexual behavior may be found more in
the brain and ovaries than in the clitoris itself.